s3:idmap_tdb2: remove the undocumented option of the silly name "tdb:idmap2.tdb"

s3:idmap_tdb2: remove the undocumented option of the silly name "tdb:idmap2.tdb"

In ancient times, when ctdb had not support for persistent databases and
tdb2 was introduced as a two-layer solution and it was more important than
today to be able to change the location of the permanent database file
because it had to reside on shared storage.

But these were times when idmap_tdb2 was not even officially released.
Nowadays, with ctdb handling the persistent idmap2.tdb database, the path
is stripped anyways, so this undocumented option has become unnecessary
and is hence removed.

Autobuild-User: Michael Adam <obnox@samba.org>
Autobuild-Date: Wed Jul 27 05:37:57 CEST 2011 on sn-devel-104
(cherry picked from commit 3276060da4e7d495bd5cf5cbf7237e64d948ee77)

The last 10 patches address
bug #8328 (final cleanup of idmap_tdb2 code and docs needed for release).
(cherry picked from commit 742b8c3c14ec8708eb5bfe5e7960c1c93ec0096c)